Ultrasensitive quantification of cancer biomarkers in complex samples is of great significance to clinical decision-making, and thus facilitates enabling early-stage diagnosis, monitoring cancer progression and evaluating therapeutic interventions. Currently available quantitative immunoassays, such as enzyme-linked immunosorbent assay (ELISA), the most popular format in clinical biomarker detection, typically measure protein biomarkers at concentrations above 0.1 ng/mL (Thaxton, C. S. et al., Proc. Natl. Acad. Sci. U.S.A. 2009, 106: 18437-18442). This detection sensitivity is unable to reach the clinical threshold of many protein biomarkers especially in the early stages of the diseases when their concentrations in clinical samples are generally in the range of fg/mL to pg/mL. It is therefore extremely important to develop quantitative immunoassays with ultra high sensitivity.
The past decades have witnessed a variety of enhanced immunoassays with ultrahigh sensitivity. Amongst these strategies, gold nanoparticle (AuNP)-based colorimetric assays have been incorporated with immunoreactions for biomarker detection, which have drawn considerable attention owing to their unprecedented sensitivity and convenient readout. Presently, AuNP-based colorimetric assays mainly depend on the monodisperse or aggregated process of AuNPs, where detection target-induced molecular events are transformed into the respective red-to-blue (or purple) color change. These clear red-to-blue (or purple) results can be easily distinguished by the naked eye, making it suitable for point-of-care (POC) diagnostics. Although the reported AuNP-based immunoassays provide sufficiently high sensitivity, most of them are unable to quantify the detection targets of interest due to their narrow linear detection ranges.
Thus, there exists an unmet need for an ultrasensitive and quantitative immunoassay.